The present invention relates to interference reduction in a cellular mobile radiotelephone system and more particularly to reduction of interference in such a system resulting from a conventional, unrestricted directed retry function.
In mobile radiotelephone systems, one of the main technological challenges is to provide a sufficiently large number of radio channels to meet system demand while at the same time minimizing interference of those channels with one another. In cellular mobile radiotelephone systems, this problem has been addressed by allocating frequencies to cells according to a frequency reuse pattern such that a specified distance, known as the frequency reuse distance, separates geographical areas using a common frequency. Referring to FIG. 1, for example, cells A.sub.1 through A.sub.7 in the lower left-hand cluster of cells each use different sets of frequencies. The same frequencies can be repeated some distance away as in the cluster to the upper right having cells B.sub.1 through B.sub.7. Cell B.sub.4 could use one or more of the same frequencies as cell A.sub.4, and so forth. Mobile stations normally served in cell A.sub.4 cause some interference at the base station and mobile stations in cell B.sub.4, and vice versa. Such interference is usually minimal and is measured by comparing the level of interference to the effective signal level at the base station or mobile station being interfered with to yield a C/I (carrier-to-interference) ratio. The technique of frequency reuse has proven effective in preventing undue interference of same-frequency channels.
A problem arises, however, when a call access is attempted at a time when all of a cell's voice channels available for the call are occupied (cell congestion). The typical response to such a situation is for the mobile switching center MSC to send a directed retry instruction to the mobile station attempting access together with a list of the frequencies of the control channels of all of the congested cell's neighboring cells. A directed retry instruction essentially informs the mobile station attempting to access the mobile radiotelephone system that access is not available through a cell just tried, but instructs the mobile stations to attempt access again through one of a number of different neighboring cells.
Since no connection of the mobile station to the radiotelephone system across a traffic channel has yet been established, there has been no substantial opportunity for the location of the mobile station relative to cell boundaries to be determined within the radiotelephone system. Such location may be determined, for example, by monitoring the strength of transmissions by the mobile station at several different cell sites. When the location of the mobile station is known, the MSC may direct the mobile station to reattempt access through a particular designated cell. Instead, the mobile station monitors reception on each of the neighboring cell's frequencies and chooses a strongest received frequency across which to reattempt access.
Given a flat topography, the frequency chosen will usually be that of the neighboring cell closest to the mobile station. Because of varying topography, however, the propagation paths and resulting signals strengths may be varied such that the strongest received channel may not always be that of the closest neighboring cell but rather may be that of a more distant neighboring cell. In this latter situation, the potential for interference increases.
A mobile station M in FIG. 1, for example, would normally access and be served by cell A.sub.7 but might be directed to cell A.sub.4 if cell A.sub.7 is congested. If the mobile station M sets up the call in cell A.sub.4, the mobile station is farther away from cell A.sub.4 than most mobile stations served by cell A.sub.4. The distance to the closest interferer, cell B.sub.4, is shorter than normal. The result is a reduced C/I ratio for the mobile station M and possibly also for a call handled in cell B.sub.4.
A further source of interference is adjacent channel disturbance which may occur when adjacent cells use two frequencies that, although different, are close to one another. Because filtering techniques do not provide perfect attenuation of adjacent frequencies, some energy at an adjacent frequency will pass through the channel filters and cause interference characterized by a C/A (carrier-to-adjacent) ratio. In FIG. 1, if the mobile station M received service from cell A.sub.4 on a frequency f as a result of directed retry, and if cell A.sub.7 used frequency f+1 or f-1 at the same time, then the mobile station would both experience and cause interference. Such interference becomes more widespread the farther toward the interior of the cell the mobile station is because the mobile station's signal is stronger over a greater portion of the cell. If the mobile is at the cell border, interference is less.
To overcome the foregoing problems it would be desirable to have the capability of treating overflow new call access requests according to the potential of the mobile station for causing disruptive interference. In particular, it would be desirable to restrict the directed retry function with respect to mobile stations near the interior of a congested cell.